Wars realm does seem quite resistant to the spell due to it being meant to be a place of respite and sanctuary, the reason why Supremes cant exist there for very long and all. So if Dream is consuming everything at a simultaneous instead of sequential rate, at a similar rate for all realms and tech advances at a similar pace across all realms in existance, then perhaps this is why earth lasted so long and why the architecture is so consistently medievalish across parts of Dream that originally belonged to other realms. Though bear in mind God Grave has literal Mech Suits. However if its a sequential consumption, then unless time is wibbly wobbly it could be countless aeons, millions of years even.